In the past, there were proposed a variety of photovoltaic elements with an organic semiconductor layer (these photovoltaic elements will be hereinafter referred to as "organic photovoltaic elements"). However, for any of those known organic photovoltaic elements, there are problems that the light collecting efficiency is extremely low and thus, the photoelectric conversion efficiency is extremely low accordingly. Because of this, research works in order to put organic photovoltaic elements to practical use have been delayed in comparison with those on pn or pin junction photovoltaic elements with a semiconductor layer comprising a crystalline or amorphous silicon.
In recent years, attention has been directed to use of an organic semiconductor film in a photovoltaic element since such organic semiconductor film can be easily formed without using a specific apparatus as in the case of forming a crystalline or amorphous silicon semiconductor film and at a reduced cost. And research works have been made in order to develop a practical usable organic semiconductor film which makes the resulting photovoltaic element to exhibit a desirably high photoelectric conversion efficiency.
Now, in Appl. Phys. Lett., 32, 495(1978), J. Chem. Phys., 71, 1211(1979) or Jpn. J. Appl. Phys. 20, Suppl. 20-2, 135 (1980), it is described that organic semiconductor materials i.e. anthracene, tetracene, merocyanine, phthalocyanine, hydroxy squarylium (OHSq in abbreviation) [see, Appl. Phys. Lett., 29, 414(1976)], chlorophyll and pyrrole are usable in photovoltaic elements, and Schottky type photovoltaic elements prepared by using these semiconductor materials provide a photoelectric conversion efficiency of about 0.2 to 1% with irradiation of AM-O spectrum light. As for the reason why these photovoltaic elements are such that merely provide such a low photoelectric conversion efficiency, it is not clear enough at the present stage, but there have been illustrated such factors that the density of a carrier trap particularly in the organic semiconductor is large; the lifetime and hole mobility of a photocarrier are small; and the diffusion length of a photocarrier is short. In addition to these, there have been illustrated other factors that the organic semiconductor hardly provides an ohmic contact since it has a large electrical resistivity, and the photoelectric conversion efficiency is decreased as the intensity of light impinged is heightened.
Now, the public attention has been focused on polysilane compounds which are expected to replace the foregoing organic semiconductor materials which are problematic when used in photovoltaic elements.
Now, in The Journal of American Chemical Society, 125, pp. 2291 (1924), polysilanes are reported to be insoluble in solvents. In recent years, since it was reported in The Journal of American Ceramic Society, 61, pp. 504 (1978) that polysilanes are soluble in solvents and films can be made of them, the public attention has been focussed on polysilanes. Japanese Unexamined Patent Publications Sho. 60(1985)-98431 and Sho. 60(1985)-119550 disclose polysilanes which can be dissociated with ultraviolet rays and utilization of them in resists. Further, Physical Review B 35, pp. 2818 (1987) discloses polysilanes having photosemiconductor characteristics in which carriers are mobile due to .sigma.-bonds of their principal chains. And these polysilanes are expected to be usable also in electrophotographic photosensitive members. However, in order that polysilane compounds be applicable in electronic materials, those polysilane compounds are required to be such that are soluble in solvents and capable of providing films which are not accompanied by minute defects and excel in homogeneity. The electronic materials should not be accompanied by any minute defects and because of this, polysilane compounds to be used in the preparation of such electronic materials are required to be high quality polysilane compounds, which can be structurally defined also with respect to substituents and do not cause any abnormality upon film formation.
There have been made various reports of synthesis of polysilane compounds. Those polysilane compounds reported are still problematic in using them in electronic materials. In The Journal of American Chemical Society 91(11), pp. 3806 (1972) and Japanese Patent Publication Sho. 63(1988)-38033, there are disclosed low-molecular weight polysilane compounds in which all the Si radicals being substituted by organic groups. Those described in the former literature are of the structure in which the end group of dimethylsilane being substituted by a methyl group. Those described in the latter literature are of the structure in which the end group of dimethylsilane being substituted by an alcoxy group. Each of them is 2 to 6 in degree of polymerization and does not exhibit characteristics as the polymer. Particularly in this respect, each of them does not have an ability of forming a film as it is and is not industrially applicable. High-molecular weight polysilane compounds of the structure in which all the Si radicals being substituted by organic groups are reported in Nikkei New Material, pp, 46, Aug. 15, 1988. These are synthesized through specific intermediates to cause reduction in their yield and it is difficult to mass-produce these on the industrial scale.
In addition, method of synthesizing polysilane compounds are reported by The Journal of Organometallic Chemistry, pp. 198 C27 (1980) and The Journal of Polymer Science, Polymer Chemistry Edition vol. 22, pp. 159-170 (1984). However, any of these synthetic method is directed only to condensation reaction of the polysilane principal chain but does not touch upon the end groups. And, is any of these synthetic methods, unreacted chlorine radicals and by-products due to side reactions are caused, and it is difficult to obtain stably polysilane compounds as desired.
Use of such polysilane compounds as described above as a photoconductive material are proposed by U.S. Pat. No. 4,618,551, U.S. Pat. No. 4,772,525 and Japanese Unexamined Patent Publication Sho. 62(1987)-269964. However, in any of these cases, occurrence of undesirable negative effects due to said unreacted chlorine radicals and said by-products caused by side reactions are considered.
In U.S. Pat. No. 4,618,551, the foregoing polysilane compounds are used in electrophotographic photosensitive members and an extremely high voltage of 1000 V is applied upon use of those photosensitive members, although a voltage of 500 to 800 V is applied in an ordinary electrophotographic copying machine.
It is considered that this is done in order to prevent occurrence of spotted abnormal phenomena on images reproduced since defects due to the structural defects of the polysilane compound will cause in the electrophotographic photosensitive member at an ordinary potential. In Japanese Unexamined Patent Publication Sho. 62(1987)269964, electrophotographic photosensitive members are prepared by using the foregoing polysilane compounds and a photosensitivity is observed for each of them. However, any of those electrophotographic photosensitive members is not sufficient in photosensitivity and is inferior to the known selenium photosensitive member or the known organic photosensitive member in any respect.
There area number of unsolved problems for any of the known polysilane compounds to be utilized in the electronic materials. At the present time, any polysilane compound which can be desirably used in the preparation of electric or electronic devices are not yet realized.